Optical and infrared properties of active galactic nuclei in the Lockman Hole^⋆

¹ Max Planck Institut für extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
e-mail: erovilos@mpe.mpg.de
² Max Planck Institut für Plasmaphysik, Boltzmannstraße 2, 85748 Garching, Germany
³ Technische Universität München, Fakultät für Physik, James-Frank-Straße, 85748 Garching, Germany
⁴ Excellence Cluster Universe, TUM, Boltzmannstraße 2, 85748 Garching, Germany
⁵ Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721, USA
⁶ Institute of Physics, Eötvös University, Pázmány P. s. 1/A, 1117 Budapest, Hungary

Received: 15 September 2010
Accepted: 23 February 2011

Abstract

We present the observed-frame optical, near-, and mid-infrared properties of X-ray-selected active galactic nuclei (AGN) in the Lockman Hole. Using a likelihood ratio method on optical, near-infrared or mid-infrared catalogues, we assigned counterparts to 401 out of the 409 X-ray sources of the XMM-Newton catalogue. Accurate photometry was collected for all the sources from U to 24 μm. We used X-ray and optical criteria to remove any normal galaxies, galactic stars, or X-ray clusters among them and studied the multi-wavelength properties of the remaining 377 AGN. We used a mid-IR colour–colour selection to understand the AGN contribution to the optical and infrared emission. Using this selection, we identified different behaviours of AGN-dominated and host-dominated sources in X-ray-optical-infrared colour–colour diagrams. More specifically, the AGN-dominated sources show a clear trend in the f_x/f_RC vs. R_C − K and f_24 μm/f_RC vs. R_C − K diagrams, while the hosts follow the behaviour of non X-ray detected galaxies. In the optical-near-infrared colour–magnitude diagram we see that the known trend of redder objects to be more obscured in X-rays is stronger for AGN-dominated than for host-dominated systems. This is an indication that the trend is more related to the AGN, which contaminate the overall colours, than to any evolutionary effects. Finally, we find that a significant fraction (~30%) of the reddest AGN are not obscured in X-rays.